Four p38 MAPK isoforms (alpha, beta, gamma and delta respectively) have been identified, each displaying a tissue-specific expression pattern. The p38 MAPK alpha and beta isoforms are ubiquitously expressed throughout the body and are found in many different cell types. The p38 MAPK alpha and beta isoforms are inhibited by certain known small molecule p38 MAPK inhibitors. Earlier generations of compounds were highly toxic due to the ubiquitous expression pattern of these isoforms and off-target effects of the compounds. More recent inhibitors are improved to be highly selective for p38 MAPK alpha and beta isoforms and have a wider safety margin.
Less is known about the p38 MAPK gamma and delta isoforms. These isoforms are expressed in specific tissues/cells (unlike the p38 alpha and p38 beta isoforms). The p38 MAPK-delta isoform is expressed more in the pancreas, testes, lung, small intestine and kidney. It is also abundant in macrophages (Smith, S. J. (2006) Br. J. Pharmacol. 149:393-404) and detectable in neutrophils, CD4+ T cells and endothelial cells (www.genecard.org, Karin, K. (1999) J. Immunol.). Very little is known about the expression of p38 MAPK gamma but it is expressed more in brain, skeletal muscle and heart, as well as in lymphocytes and macrophages (www.genecard.org).
Selective small molecule inhibitors of p38 MAPK-gamma and -delta are not currently available, but one existing compound, BIRB 796, is known to have pan-isoform inhibitory activity. The p38 gamma and p38 delta inhibition is observed at higher concentrations of the compound than those required to inhibit p38 MAPK alpha (Kuma, Y. (2005) J. Biol. Chem. 280:19472-19479). BIRB 796 also impaired the phosphorylation of p38 MAPKs or JNKs by the upstream kinase MKK6 or MKK4. Kuma discussed the possibility that the conformational change caused by the binding of the inhibitor to the MAPK may affect the structure of both its phosphorylation site and the docking site for the upstream activator, therefore impairing the phosphorylation of p38 MAPKs or JNKs.
p38 MAP kinase is believed to play a pivotal role in many of the signalling pathways that are involved in initiating and maintaining chronic, persistent inflammation in human disease, for example, severe asthma and COPD. There is now abundant literature which demonstrates that p38 MAP kinase is activated by a range of pro-inflammatory cytokines and that its activation results in the recruitment and release of further pro-inflammatory cytokines. Indeed, data from some clinical studies demonstrate beneficial changes in disease activity in patients during treatment with p38 MAP kinase inhibitors. For instance Smith, S. J. (2006) Br. J. Pharmacol. 149:393-404 describes the inhibitory effect of p38 MAP kinase inhibitors on TNFα (but not IL-8) release from human PBMCs. Use of inhibitors of p38 MAP kinase in the treatment of chronic obstructive pulmonary disease (COPD) is proposed. Small molecule inhibitors targeted to p38 MAPKα/β have proved to be effective in reducing various parameters of inflammation in cells and tissues obtained from patients with COPD, who are generally corticosteroid insensitive, (Smith, S. J. (2006) Br. J. Pharmacol. 149:393-404) and in vivo animal models (Underwood, D. C. et al. (2000) 279:895-902; Nath, P. et al. (2006) Eur. J. Pharmacol. 544:160-167; Medicherla S. et al. (2008) J. Pharm. Exp. Ther. 324:921-929). Irusen and colleagues also suggested the possibility of involvement of p38 MAPK α/β on corticosteroid insensitivity via reduction of binding affinity of glucocorticoid receptor (GR) in nuclei (Irusen, E. et al., (2002) J. Allergy Clin. Immunol., 109:649-657). Clinical experience with a range of p38 MAP kinase inhibitors, including AMG548, BIRB 796, VX702, SCIO469 and SCIO0323 is described in Lee et al. (2005) Current Med. Chem. 12, :2979-2994.
COPD is a condition in which the underlying inflammation has been reported to be substantially resistant to the anti-inflammatory effects of inhaled corticosteroids. Consequently, an effective strategy for treating COPD may well be to develop an intervention which both has inherent anti-inflammatory effects and is able to increase the sensitivity of lung tissues from COPD patients to inhaled corticosteroids. The recent publication of Mercado et al. (2007) American Thoracic Society Abstract A56) demonstrates that silencing p38 gamma has the potential to restore sensitivity to corticosteroids. Thus there may be a “two pronged” benefit to the use of a p38 MAP kinase inhibitor for the treatment of COPD and severe asthma.
There is now a substantial body of evidence which strongly implicates the role of respiratory viral infections in initiating exacerbations in patients suffering from asthma and/or COPD. Exacerbations require an increase in treatment intensity to re-establish control of disease symptomology. If severe, exacerbations may well result in the hospitalisation or, at its most extreme, the death of patients. Those viruses which are commonly associated with exacerbations include rhinovirus, influenza and respiratory syncitial virus. The cellular responses to these viruses are now known to include the up-regulation of ICAM1 (intercellular adhesion molecule 1) and release of cytokines, as well as the replication of virus particles. There has been some investigation into the effect of p38 MAP kinase inhibitors on these viral responses and some reports suggest that protective effects can be detected with p38 MAP kinase inhibitors. In particular, some reports suggest that inhibition of virus-induced release of IL-8 can be achieved in vitro with the known compound, SB203580. It is noteworthy that the assessment of an agent's efficacy to reduce rhinovirus-induced inflammation and virus replication in preclinical in vivo models remains a significant challenge. However, there are well-established in vivo models of influenza in the mouse and parainfluenza in the guinea pig.
The major obstacle hindering the utility of p38 MAP kinase inhibitors in the treatment of human chronic inflammatory diseases has been the toxicity observed in patients. This has been sufficiently severe to result in the withdrawal from clinical development of many of the compounds progressed, including all those specially mentioned above.
There remains a need to identify and develop new compounds therapeutically useful as p38 MAP kinase inhibitors which have improved therapeutic potential, in particular which are more efficacious, longer acting and/or less toxic at the relevant therapeutic dose. An objective of the present invention is to provide a compound which inhibits p38 MAP kinase, for example with certain sub-type specificity, which shows good anti-inflammatory potential.